The present invention relates to an idle speed control device suitable for use as an electronic control actuator which operates to set the idle speed of an automotive engine automatically at a desired speed in response to change in the cooling water temperature or the ambient air temperature. More particularly, the invention is concerned with an idle speed control device of the type mentioned above, having an improved construction of the flow-rate controlling portion thereof.
An actuator has been known which is designed for automatically controlling the idle speed of automotive engine in response to a change in the cooling water temperature or in the intake vacuum. This known actuator has a flow rate controlling section which includes a body defining a passage of air to be controlled, a pair of seats formed on an intermediate portion of the body, and a pair of metering valves fixed to the rod of an electromagnetic actuator. An example of this type of idle speed control device is shown in the U.S. Pat. No. 4,314,585. The actuator, which is constituted by an electromagnetic driving portion for converting an electric input into a mechanical output and the flow rate control section mentioned above, is adapted to be controlled by a processing circuit which performs a predetermined computation upon receipt of signals from a water temperature sensor and a crank angle sensor, in such a manner as to control the flow-rate of bypass air such as to maintain a desired engine speed.
Thus, the actuator conducts an automatic and continuous control such as to maintain the idle speed at a predetermined speed, upon sensing the cooling water temperature and the engine speed.
As mentioned before, the known actuator has a flow-rate control section which is constituted by a pair of seats and a pair of metering valves adapted for cooperation with these seats. In this flow-rate control section, for the reason concerning the assembly, one of the seats has a diameter greater than that of the other. Since the cross-sectional area of passage between one seat and the cooperating valve and that between the other seat and the associated valve differ from each other, the vacuum forces determined by such cross-sectional areas differ from each other, and the vacuum forces tend to be inverted at an intermediary.
As will be understood from the foregoing statement, the flow-rate characteristics of the conventional actuator is liable to be affected by the pressure differential across the metering valves. More specifically, as shown in FIG. 10, the flow-rate characteristic curve a as obtained when the intake vacuum is -500mmHg crosses the curve b showing the flow-rate characteristics as obtained when the intake vacuum is -600mmHg, at an intermediate level of the electric input. Namely, selecting the flow-rate characteristic curve a as the standard or reference, the flow rates obtained at different intake vacuum level expressed by the curve b is smaller than the reference value when the electric input is rather small but becomes greater than the reference value when the electric input is rather large.
Thus, the flow-rate characteristics tend to be inverted at an intermediate level of the electric input when the pressure differential is large, so that a complicated controlling software is required.
It is to be noted also that the levels of vacuum force which act on both sides of a pair of metering valves are not equalized. Namely, the vacuum force acting on one end of the pair of metering valves is greater than that acting on the other end. In consequence, a vacuum force as a disturbance is applied to the metering valve in addition to the electromagnetic force. Thus, the input/output characteristics are affected by the pressure differential across the pair of metering valves, as will be seen from FIG. 11.
The conventional actuator involves a problem in that the initial leak is large particularly in the inoperative state, i.e., when the electric input is zero. Namely, in the flow-rate controlling portion which is constituted by a pair of valves and cooperating seats, it is extremely difficult to make the distance between two seats precisely coincide with the distance between two valves. Therefore, the close contact between the valve and the seat is failed in either one of the combination of the valve and the seat, so that a certain rate of initial leak is unavoidable. A large initial leak makes it impossible to set the idle speed at a low level. This is quite inconvenient from the view point of development of fuel saving and silent engine.